Miscellaneous Logging

This page has some old data logging graphs I had on my website years ago.

I have some Dallas Semiconductor DS1820 Onewire temperature sensors (the
old ones, before they realised they had a design flaw and brought out the
DS18S20 instead). They're interfaced via the serial port to a PC (running DOS), and logged with software I got off the web.
The sensors are in a transistor-sized (TO92) package , have inbuilt
analog to digital conversion, serial data output, and can work off parasite
power. Although they're not wonderfully cheap, they do give digital temperature readings
with virtually no effort - from this perspective, they're pretty good value for money.

Dallas provide a schematic for a serial port interface (providing power &
comms), using 4 diodes & a resistor. This simple interface has some
limitations on cable length etc, but it's not a problem for basic setups.
Multiple sensors can be attached to the common buss and
addressed individually.

The variety I have are calibrated to 0.5 degrees
Centigrade, but the resolution is closer to a hundredth of a degree if you do
an extra read of the internal registers. The high resolution makes them quite
good for showing small temperature fluctuations, even if the absolute
measurement is a little off. The temperature conversion (to digital) takes
about 200ms.

The following plot shows the temperature at my work, over a 2.5 day period. I
setup a sensor about a foot above the floor, one about halfway up a wall and
the last one about a foot below the ceiling. Readings from each of the sensors
were logged at 1 minute intervals. The log file (plain text of temperatures
with timestamps) was imported into Excel to produce the graph. (this was
all before I started using linux)

You can see when the sun hits the building in the morning & the temperature
starts to rise. When I leave the building at night (I'm normally last to go)
the temperature starts to fall, looking very much like a typical exponential
decay. From 7pm to 8pm in the middle of the plot, I was out of the office and
the temperature fell - when I arrived back, the ceiling temperature rose by
most of a degree until I left. Presumably this rise is from a combination of:

PC & monitor coming out of standby - maybe 200watts change.

me - maybe 60w-100w (estimates from various places on the web, eg.
here)

fluourescent lights - maybe 45w of heating.

Next time I do some logging, I'd like to have an outside air temperature sensor
as well.

Heater Thermostat

My 1kW oil-column heater has a simple on-off type of thermostat, as is obvious
in this graph. The red line is from a sensor clamped to one of the fins of the
heater; the pink line is the air temperature in the room, about a metre from
the heater; and the blue line is the temperature in an adjacent room, just to
show that ambient temperature in the rest of the house is fairly constant
throughout the test.

The heater temperature in this case is ranging over about 13 degrees C -
assuming my thinking is straight, this is a combination of two things:

the hysteresis in the thermostat.

how much the heater manages to heat up before the temperature rise is noticed at the thermostat.

If the heater had a better thermostat system (eg. proportional control), then
the heater would be on pretty much full power until about the 20 to 25 minute
mark, then on reduced power, supplying just enough heat to make up for the heat
which is leaking out of the room (through windows, doors, etc.)

What I'd like to test next time:

run the test for longer, to see where the room temperature ends up.

record the state of the thermostat, to see exactly when it turns on and off.

Heater Thermal Resistance

This test shows operation of the heater in a room which is too big for it to
have much heating effect. The heater is on full power for the whole time and
the room never warms enough for the thermostat to come into action.

From the graph, we can see that the heater is about 44 degrees (Celsius) hotter than
the room temperature. Since we know that the heater is supplying 1kW of heat
(nominally), we can calculate the thermal resistance of the heater, as being 44
degrees divided by 1kW = 0.044 degrees per watt.

This is a measure of the effectiveness of the heater in transferring heat to
the air - a lower number is better. Making the heater more effective won't
make the room any warmer in the long run, or save any power, but it will mean
that the heater runs at a lower temperature, and therefore be safer to touch.

Things which could improve the effectiveness:

have more fins on the heater (a greater surface area).

paint the heater black instead of white (improved heat radiation characteristics - won't make much difference at these temperatures).

maybe change the heater surface texture to increase surface area.

change the shape of the heater fins to improve airflow.

I have no definite explanation for why the heater temperature
fluctuates so much in the later part of the test. I suspect that the sensor
may not have been firmly clamped to the fin.

Letterbox Temperature

I borrowed a basic thermal data logger from a friend and stuck it in the
letterbox for 5 days, logging temperature every minute. Surely everybody wants
to know how hot it gets in the letterbox? OK, I admit that it's not very
useful.

The thick red line is the average temperature. Typical peak ambient
temperature was probably about 18-20 degrees C. We can see that sunlight hits
the letterbox about 8am at this time of year, and the temperature climbs
rapidly. What I find most interesting is the temperature on the Monday - it
starts climbing about 4am. The sun definitely wasn't up at that time of the
morning, but presumably there was a warm breeze.

(The average line is derived from slightly more data points than are shown
here, hence the slight discontinuity just before 5am)

Car Boot Temperature

This is probably more useful than logging the letterbox temperature, but not by
much. Two probes were used in the car boot (trunk). One on the floor of the
boot, just above where the exhaust pipe runs - that's the nice hot red line.
The other probe (blue) was about mid-way up one side of the boot. The third
line (green) is the car speed (logged with GPS) in units of 10kmh, to fit in
neatly with the temperature scale - it's only really there to show when the car
is running. The logger used has a maximum temperature of about 42 degrees C,
so the red plot is clipped at its peak.

Unsurprisingly, the temperature above the exhaust climbs rapidly once the car
is started, and starts falling rapidly about 20 minutes after the engine is
stopped. The temperature peak on the blue line is partly from heat radiating
from the floor of the boot, but is mainly from being parked in the sun.

The car is a 1990 Mazda MX5 (Miata / Eunos Roadster).

GPS Logging

In 1999 I bought an OEM GPS module from a surplus dealer. The
module was meant for car navigation, so it's somewhat appropriate that I've installed it
in my car.

The GPS produces a serial datastream containing current latitude & longitude,
as well as current speed and various other things, updated once per second. My
logger hardware displays these variables and stores them to 32k EEPROM at
configurable intervals. There's capacity for 1638 readings - about 27 minutes
if logging at full rate. Most of the time, for recording my day-to-day
driving, I have it set to log every 30 seconds, giving capacity of about 13.6
hours - one or two weeks of travel, depending on how much driving I'm doing.

This plot shows activity for a fortnight, mostly travelling to work and back,
but also various social & shopping trips. What is it good for? Very little, but it looks arty.

GPS path - MX5, Snells Beach

This plot shows where we (MX5 club) went for a drive to Snells Beach for
lunch. Not the most direct route from A to B, but that's not why we were doing
it. Who wants to drive on a boring straight motorway when you can have nice
twisty roads instead?

I was logging coordinates every 5 seconds, so the plot has quite good
resolution - certainly good enough for display at this scale.